The overall goal of this project is to define the human T cell responses to influenza infection and to trivalent inactivated influenza vaccines (TIV) in younger and older adults. We hypothesize that (1) human T cell responses in younger adults will be greater than those in older adults to vaccination and to infection, (2) human CD4+ and CD8+ T cell responses will be lower to vaccine than to infection and (3) the TlV-induced T cell responses will correlate with the amount of internal proteins in the individual licensed vaccines. Influenza A virus hemagglutinin (HA) is a target of protective neutralizing antibodies, which are subtypespecific and vulnerable to antigenic drift. CD4+ and CD8+ T cell responses are thought to be more subtype cross-reactive. It is clear from studies in mouse models of influenza A virus infections that T cells can provide a second important line of defense, especially in the face of marked antigenic drift or shift due to emergence of viruses with changes in HA antibody combining sites, and there are also limited clinical studies which suggest the importance of T cells for protection, especially in a high-risk elderly population. There is, however, only a limited amount of data available on human T cell responses to influenza infection or vaccination. Importantly there appears to be more subtype cross-reactivity among influenza A virus T cell epitopes than to the antibody epitopes on HA. At present limited data suggest that current TIVs induce low to moderate CD4+ and CD8+ T cell responses. However, we found some individuals with high T cell responses to TIV. Recently, we and the other group also found the amount of influenza internal proteins in the TIVs differs. Despite the recommendation and use of about 100 million vaccine doses per year in the US alone, very little is known about TIVs induction of T cell responses and nothing is known about their contribution to vaccine associated protection. In Aim 1 we propose to analyze CD4+ and CD8+ T cell responses to TIV vaccination in younger and older adults. In these studies, we will also compare the three US-licensed TIVs for their ability to stimulate CD4+ and CD8+ T cell responses and for protection (in older adults). In Aim 2 we propose to characterize CD4+ and CD8+ T cell responses to natural influenza infections in younger and older adults and compare them to the CD4+ and CD8+ T cell responses induced by TIV. These analyses may lead to approaches towards improved influenza vaccines, which can protect against new and emerging influenza virus infections including H5N1 and other non-human strains.